Bacteroides fragilis

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Many tiny living things live in your gut. One is called Bacteroides fragilis. These are rod-shaped bacteria. They do not move on their own. Instead, they use tiny hairs to hold on.

These bacteria are very helpful. They live in the human colon. They help you get power from your food. They break down sugars like glucose and fructose. This process makes fatty acids. These acids give your body energy. Without gut bacteria, animals need 30% more food to stay healthy.

Bacteroides fragilis is also very tough. It can live in places with bile salts. It uses special proteins to stay safe. It can also handle oxygen. Most of its kind cannot live with oxygen. This helps other tiny microbes survive too.

Most of the time, these bacteria are good. But they can cause sickness if they move. If they enter the blood or tissue, they can cause infections. They can also make abscesses, which are pockets of infection. Some types may even cause swelling in the gut.

Bacteroides fragilis is a tiny living thing that lives in your gut. It is a rod-shaped bacterium that stays in the human colon. Most of the time, it is a helpful neighbor to your body. This is called being a commensal organism. It helps your body work in many important ways. It helps with your nutrition and your immune system.

This bacterium has a very special way of working. It eats many different kinds of sugars like glucose and fructose. It breaks these down through a process called fermentation. This creates fatty acids that give your body energy. It also breaks down large molecules into smaller ones for other microbes. These microbes can then use those small pieces too. This helps the whole gut community stay healthy.

Scientists have learned a lot about these tiny cells. They used to group many different types together as one subspecies. Now, experts use DNA studies to tell them apart. These studies show they are actually distinct species. B. fragilis is the most common one in its group. It makes up 41% to 78% of the isolates in that group.

B. fragilis is also very tough and can survive hard jobs. It uses enzymes to resist the effects of bile salts. It also has proteins to protect it from oxygen. Most of its kind cannot live with oxygen at all. This bacterium uses an enzyme called cytochrome bd oxidase to use up oxygen. This helps other microbes survive in the low-oxygen environment. It can even resist many different kinds of medicine.

Even though it is helpful, it can sometimes cause trouble. If it moves into the blood or tissue, it can cause infection. It is involved in 90% of anaerobic peritoneal infections. It can also cause abscesses, which are small pockets of infection. Some types might even cause inflammation in the gut. However, it also has a special part called PSA. This part can help protect animals from diseases like asthma.

Bacteroides fragilis is a specific type of bacterium found in the human colon. It is an anaerobic, Gram-negative organism, which means it thrives in environments without oxygen. Most of the time, it lives as a commensal organism. This means it lives in the human gastrointestinal tract without causing harm. In fact, it is essential for healthy gut function. It helps with mucosal immunity and supports host nutrition. However, it can become a pathogen if it is displaced. This often happens through surgery, disease, or physical trauma.

Physically, these bacteria are quite small. They are rod-shaped or pleomorphic, meaning they can change shape. Their size typically ranges from 0.5 to 1.5 micrometers in width. They are immotile because they lack flagella or cilia. Instead, they use peritrichous fimbriae to stick to molecular structures. To survive the harsh gut environment, they use surface proteins and lipopolysaccharide chains. They also release outer membrane vesicles. These tools help them navigate a volatile micro-environment.

Metabolically, B. fragilis is a highly efficient chemoorganotroph. This means it gets energy by breaking down organic compounds. It performs fermentation on many different glycans. These include common sugars like glucose, sucrose, and fructose. The bacterium also breaks down complex biopolymers and glycoproteins. It turns these into smaller molecules for other microbes to use. This process creates fatty acids through carbohydrate fermentation. These fatty acids serve as a vital energy source for the human host. Interestingly, animals without gut bacteria require 30% more calories to maintain their weight.

Surviving in the gut requires advanced environmental-sensing systems. B. fragilis uses several mechanisms to handle threats. It secretes bacteriocin proteins to reduce competition with other microbes. It also uses an enzyme called bile salt hydrolase. This enzyme helps it resist the degrading effects of bile salts. Without this, bile salts could damage or collapse the bacterial membrane. To handle oxygen, it uses proteins like catalase and superoxide dismutase. These protect the cell from harmful oxygen radicals. It also uses cytochrome bd oxidase to consume oxygen. This allows other obligate anaerobes to survive in a low-oxygen space.

This bacterium is also known for its high level of antibiotic resistance. This resistance is mainly due to its genetic plasticity. The genus Bacteroides has some of the highest resistance rates among anaerobic bacteria. Specifically, B. fragilis carries the cfiA gene. This gene encodes a metallo-beta-lactamase, which is a type of carbapenemase. This enzyme can hydrolyze and inactivate beta-lactam antibiotics like carbapenems. Species in this group show increasing resistance to drugs like clindamycin and metronidazole. These resistance genes can even be transferred to other bacteria in the gut.

Historically, scientists have changed how they classify these organisms. The B. fragilis group was once seen as different subspecies. These included B. f. ssp. fragilis and B. f. ssp. vulgatus. However, DNA homology studies led to a reclassification. Scientists now recognize them as distinct species. B. fragilis is the most common member of its group. It accounts for between 41% and 78% of all isolates in that group. Even though it is common in the group, it only makes up 0.5% of the bacteria in human stool.

Despite its helpful roles, B. fragilis can cause serious disease. It is involved in 90% of anaerobic peritoneal infections. It can be found in blood, wounds, and brain abscesses. This pathogenicity is partly due to its capsular polysaccharide. This layer protects the bacteria from phagocytosis and helps form abscesses. Some strains, called enterotoxigenic B. fragilis, are linked to chronic inflammation. This inflammation can lead to DNA damage and colon tumorigenesis. Untreated infections can have a mortality rate of 60%.

Interestingly, the bacterium also has anti-inflammatory properties. It possesses a specific molecule called polysaccharide A, or PSA. In studies, PSA has protected animals from diseases like colitis and asthma. When PSA colonizes the gut mucosa, it induces regulatory T cells. It also works to suppress pro-inflammatory T helper 17 cells. This shows that the bacterium plays a complex role in balancing the immune system. It can act as both a potential threat and a protective partner.